An activated carbon is made from carbon materials such as carbonized coconut shell, petroleum coke or coal coke that is activated to have a porous structure. The activated carbon, which is porous and thus has a large surface area, has been widely used as an absorbent, a catalyst support, and an electrode material for double layer capacitors and lithium secondary batteries. In particular, in order to increase the energy density, i.e., capacitance in an electric double layer capacitor, which may be used in a hybrid car or the like, an activated carbon having fine pores effectively formed thereon, a high crystallinity and a large surface area has been demanded to be used as an electrode material for the capacitor.
For the industrial production of such an activated carbon with effectively formed fine pores that can be used as an electrode material of an electric double layer capacitor, a method for activation has been generally used, in which a carbon material such as petroleum coke and an alkali metal compound such as potassium hydroxide are heated at a temperature of 600 to 1200° C. in an inert gas atmosphere to allow the alkali metal to ingress between and react with the resulting graphite crystal layers (Patent Literature 1). In this type of activation, the alkali metal enters the layered structure wherein condensed polycyclic hydrocarbons are layered, and as the result forms fine pores.
In a method for producing an activated carbon for an electric double layer capacitor electrode by activating a carbon material with an alkali activator, the activator is mixed in an amount of usually 2 to 4 times more by mass ratio of the carbon material. In particular, when the target specific surface area is large ranging from 2000 to 3000 m2/g, the activated carbon is produced to be also increased in “activator/carbon material” ratio. However, since the ratio of the alkali activator accounting for the production cost is large, the ratio of use of the alkali activator is required to be as low as possible.
In the case of mixing a carbon material and an alkali activator, the carbon material is water-shedding and thus poor in wettability with the alkali activator, which is water-soluble. Simple mixture of the activator and the carbon material leads to insufficient contact therebetween and thus a large amount of the activator is not used for the reaction. The resulting product (activated carbon) is, therefore, small in specific surface area.
Methods for strongly bringing the alkali activator into contact with the carbon material are known, wherein they are mechanically mixed with a ball mill or a Henschel mixer and wherein the alkali activator is mixed in a melting state (Patent Literature 2). However, in any of the methods, the use of an activator in an amount of more than the theoretical amount is required to promote the reaction efficiently but causes the cost increase.